Friends: Whatever their name/nickname is, I do often address them as "sir" as well. Sometimes I toss Japanese honorfics around as well.
College staff: First name. Through my experience college tutors hate being called Mr/Mrs/Miss anything.
Customers: "Mate" most of the time
Total strangers: My mummy says I mustn't talk to strangers. But I never listen to what she says anyway so it's usually nothing; I just lock eyes and say "Excuse me..." and go.
Third person: Guy, or lady.